This invention relates to the interfacial preparation of aromatic polycarbonates, and more particularly to their preparation by a continuous method.
A well known method for polycarbonate preparation employs at least one dihydroxyaromatic compound, typically a bisphenol; a carbonyl halide, typically phosgene; a base, typically an alkali metal hydroxide, as an acid acceptor; a catalyst; and a substantially inert, substantially water-insoluble organic liquid such as methylene chloride. This method is frequently designated "interfacial" by reason of the interface existing between the distinct aqueous and organic phases.
Interfacial polycarbonate preparation has, by and large, been a batch process although numerous disclosures of continuous processes exist in the art. For example, U.S. Pat. No. 4,737,573 discloses a method adapted to continuous operation, wherein the first step is preparation of an intermediate product comprising polycarbonate oligomers, typically containing chloroformate end groups, in a tank reactor such as a continuous stirred tank reactor (CSTR). This intermediate product may then be converted to high molecular weight polycarbonate in one or, if necessary, two downstream reactors. The first such reactor may be another CSTR and the second may be a limited back-mixing reactor; i.e., one approaching plug flow conditions.
One advantage of batch operation over the years has been the freedom to change grades of polycarbonate manufactured by merely varying batch parameters. This is no longer as important a factor as it was, however, by reason of the alternative offered by redistribution according to U.S. Pat. No. 5,414,057; i.e., a single grade of polycarbonate can be converted by redistribution into other grades of varying molecular weights. Thus, continuous operation becomes more attractive as a commercial possibility.
Copending application Ser. No. 07/724,642 discloses a method for interfacial polycarbonate preparation which also involves an initial step of oligomer formation. The proportion of base (e.g., alkali metal hydroxide) employed therein is preferably equal to 2+4Y(Z-1) moles of base per mole of dihydroxy compound, wherein Y is in the range of about 0-0.5 and Z is the number of "moles of carbonic acid derivative per mole of dihydroxy compound"; i.e., the molar ratio of carbonyl halide to dihydroxyaromatic compound. By employing base in this proportion, it is allegedly possible to produce an oligomer intermediate having a weight average molecular weight (Mw) in the range of about 1,500-8,000 preferably about 1,500-4,000, which offers significant advantages.
U.S. Pat. No. 5,359,117 discloses a continuous method for interfacial polycarbonate preparation in which an essential feature is maintenance of the ratio of equivalents of base to dihydroxyaromatic compound in the range of 1.15-1.6. Such a ratio allegedly produces polycarbonate having a well regulated molecular weight distribution.
The molecular weight of the intermediate oligomer product unquestionably also has a significant effect on the properties of the high molecular weight polycarbonate ultimately formed in a continuous process. It has been found that if the oligomer molecular weight is too low, phosgenation efficiency may decrease, increasing raw material costs and necessitating very efficient scrubbing procedures to avoid discharge of very toxic phosgene into the atmosphere. In addition, low molecular weight oligomers tend to be less efficient at dissolving any unreacted dihydroxyaromatic compound, affording mixtures containing a solid phase which is difficult to process. On the other hand, oligomers of unduly high molecular weight produced in a continuous process can produce thermodynamically unstable polycarbonate products.
Therefore, there is a need for a continuous polycarbonate preparation method which is capable of producing an acceptable balance of phosgenation efficiency and thermodynamic stability in the final product.